The present invention relates to electronic trip units for circuit breakers, and more particularly to a self-diagnostic check of a microprocessor-based electronic trip unit.
In electrical power distribution systems, circuit breakers have been used to terminate the flow of current in the event of a fault in the system. Traditionally, in circuit breaker applications, trip units have been thermally based or magnetically based, and perform well for their intended function, which will be appreciated by one of ordinary skill in the art. More recently, interest in microprocessor-based electronic trip units has grown. Circuit breaker trip units having microprocessor controls are being implemented in increasing numbers in power supply circuit breakers. These microprocessor-based trip units replace the traditional thermal and magnetic trip units to initiate automatic circuit interruption by the circuit breaker in response to electrical fault conditions. The popularity of microprocessor-based circuit breakers is largely due to their versatility. That is, a microprocessor-based trip unit provides convenient trip definition and setting adjustment that is precisely tailorable to a particular application.
Typically, the trip units are mounted within a recess in a circuit breaker enclosure or housing. Trip actuating and input/output (I/O) signals are transmitted between the trip unit and electronic devices within the circuit breaker through pin and socket connections between the trip unit and circuit breaker. Typically, these pin and socket connections are made at the bottom of the recess into which the trip unit is installed. Because the connection is made at the bottom of the recess, it is impossible for the technician to see if the connection is properly made. In other words, the interface between the trip unit and the circuit breaker is a xe2x80x9cblind interfacexe2x80x9d.
Typically, the current sensing devices within the circuit breaker sense current within the protected circuit. The sensed signal is provided by way of the pin and socket connection to the trip unit. The trip unit analyzes the sensed signal for one or more indications of fault within the protected circuit. Such indications include: time-delayed-over-current, instantaneous-over-current, over-voltage, under-voltage, over-frequency, under-frequency, over-power, volt-to-current-mismatch, etc. The list of potential fault indications is extensive and within the purview of those skilled in the art to select and design for a given application. If the trip unit detects one of these fault indications, the trip unit provides a trip signal by way of the pin and socket connections to a trip actuator within the circuit breaker. The trip actuator actuates an operating mechanism that causes a pair of main current carrying contacts within the circuit breaker to open, thus stopping the flow of electrical current in the protected portion of the distribution circuit.
A improper connection of any pin and socket could cause the trip unit to receive an erroneous sensed signal from the current sensors or to provide an erroneous signal to the trip actuator, thus increasing the frequency of nuisance trips. Such improper connections may be caused by not fully inserting the pins in the pin sockets or by misalignment of the pins with the pin sockets. Unfortunately, the improper installation of a pin and pin socket may not be detected until a nuisance trip occurs.
In an embodiment of the present invention, a trip unit for electrically connecting to a circuit breaker includes a microprocessor circuit and a terminal electrically connected to the microprocessor circuit. A power source is electricaly connected intermediate the terminal and the microprocessor circuit. The terminal is electrically connected to ground when the terminal is connected to the circuit breaker.
When the terminal is disconnected from the circuit breaker, the terminal is disconnected from ground and the power source provides an input signal to the microprocessor circuit.
In response to the input signal, the microprocessor unit provides an output signal indicating that the trip unit has been disconnected from the circuit breaker.